Liquid cooled power electronic circuit comprising a stacked array of directly cooled semiconductor chips

ABSTRACT

A stacked array of channeled semiconductor chips defining a power electronic circuit is mounted in a sealed container provided with inlet and outlet passages for liquid coolant. Leadframe terminals supported by the container engage selected terminals of the semiconductor chips and form leads for mounting the container on a circuit board having electrical and fluid interconnects.

TECHNICAL FIELD

The present invention relates to liquid cooling of power semiconductor electronics, and more particularly to a liquid cooled power electronic circuit defined by a stack of electrically interconnected integrated circuit chips.

BACKGROUND OF THE INVENTION

Various types of cooling mechanisms can be used to remove waste heat from high power semiconductor devices such as power FETs and IGBTs. In cases where the waste heat and/or the ambient temperature are very high, the power device packages can be mounted on a liquid-cooled heat exchanger or a cold plate through which liquid coolant is circulated. The heat transfer can be significantly improved by bringing the liquid coolant directly into contact with the semiconductor chip (die), as shown in the Patent Application Publication Nos. 2006/0022334; 2006/0034052; 2006/0291164; and 2007/0063337, all assigned to Delphi Technologies, Inc. As described in these patent documents, a major surface of the semiconductor chip (say, the drain terminal of a power FET) can be undercut to define an array of fluid conducting channels through the bulk region of the chip, and the chip can be packaged so that some or all of the circulating fluid flows through the channels to remove heat from the chip. As described in the aforementioned Publication No. 2006/0022334, for example, the direct die cooling approach can be implemented on a modular or stand-alone basis by packaging a channelled semiconductor chip in a liquid cooled container, and mounting the container on a circuit board with both electrical and fluid interconnects.

SUMMARY OF THE INVENTION

The present invention is directed to an improved direct die cooling arrangement in which a stacked array of channeled semiconductor chips is mounted in a sealed container provided with inlet and outlet passages for liquid coolant. Juxtaposed terminals of the semiconductor chips are electrically joined to form a power electronic circuit. Leadframe terminals supported by the container engage exposed terminals of the semiconductor chips and form leads for mounting the container on a circuit board having electrical and fluid interconnects.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A depicts an H-Bridge transistor circuit; and FIG. 1B is a side view of a stacked semiconductor die assembly for effectuating the circuit of FIG. 1A according to this invention.

FIG. 2A depicts a ½-H-Bridge transistor circuit; and FIG. 2B is a side view of a stacked semiconductor die assembly for effectuating the circuit of FIG. 2A according to this invention.

FIG. 3 is a top view of a liquid cooled power semiconductor circuit package based on the stacked semiconductor die assembly of FIG. 1B, but with the cover of the package removed.

FIG. 4 is an exploded pseudo-cross-sectional diagram of the liquid cooled power semiconductor circuit package of FIG. 3.

FIGS. 5A, 5B and 5C respectively depict first, second and third configurations of a two-die stacked die assembly.

DESCRIPTION OF THE PREFERRED EMBODIMENT

In general, the present invention is directed to a semiconductor packaging approach in which a plurality of semiconductor power chips (die) are interconnected by stacking in a liquid cooled container to form a direct die cooled power electronic circuit. The invention is primarily disclosed in the context of power field-effect transistors (FETs) configured to form an H-bridge or ½-H-Bridge power transistor circuit, but it will be recognized that the disclosed approach equally applies to other semiconductor power devices and other power electronic circuits.

FIGS. 1A and 1B respectively illustrate an H-Bridge transistor circuit and a stacked assembly or array of power-FET chips 12 a-12 d for effectuating the H-Bridge circuit. Referring to FIG. 1A, the letter M designates a two-terminal (a and b) electrical load such as a DC motor. The FETs 12 b and 12 c respectively couple load terminal a to power (V+) and ground (GND); and the FETs 12 a and 12 d respectively couple load terminal b to power (V+) and ground (GND). The source, drain and gate terminals of FETs 12 a-12 d are designated by the letters S, D and G, respectively. In operation, load current in a first direction (a-to-b) is established by turning on FETs 12 b and 12 d with FETs 12 a and 12 c turned off; and load current in the opposite direction (b-to-a) is established by turning on FETs 12 a and 12 c with FETs 12 b and 12 d turned off.

In FIG. 1B, the reference numeral 10 generally designates a stacked assembly of four power-FET chips 12 a-12 d corresponding to the four FETs 12 a-12 d in the circuit diagram of FIG. 1A. For convenience of description, the letters a-d are appended to the reference numerals designating various elements of the FET chips 12 a-12 d. Each FET 12 a-12 d has a first major surface on which are formed the source and gate terminals 14 a-14 d and 16 a-16 d, and a second major surface on which is formed the drain terminal 18 a-18 d. In each case, the drain terminal 18 a-18 d and the underlying inactive bulk material of the FET are partially recessed by an etching or sawing process to define a number of parallel channels 28 a-28 d separated by intervening walls 30 a-30 d. Additionally, each of the FET chips 12 a-12 d has a metal interposer sheet 32 a-32 d of copper or Alloy 42 that is preferably attached by soldering at the wafer level (i.e., prior to die singulation). The metal interposer sheets 32 a-32 d facilitate electrically testing of the singulated transistor chips 12 a-12 d, and chip-to-chip interconnections when they are stacked and electrically joined to define a power electronic circuit.

When the transistor chips 12 a-12 d are stacked as shown in FIG. 1B, the juxtaposed terminals and interposer sheets are soldered together; i.e., interposer sheet 32 a is soldered to the source terminal 14 b, interposer sheet 32 b is soldered to source terminal 14 c, and interposer sheet 32 c is soldered to interposer sheet 32 d. The lateral staggering of the chips 12 a-12 d leaves the gate terminals 16 a-16 d exposed, and affords electrical connection to selected ones of the source terminals 14 a-14 d and drain terminals 18 a-18 d.

It can be demonstrated that the assembly 10 effectuates the H-Bridge circuit of FIG. 1A. Starting at the top of the assembly 10, load terminal b is coupled to drain 18 a, V+ is coupled to sources 14 a and 14 b, load terminal a is coupled to drain 18 b and source 14 c, GND is coupled to drains 18 c and 18 d, and load terminal b is coupled to source 14 d. As explained below in reference to FIGS. 3-4, the liquid cooled container into which the stacked assembly 10 is installed includes leadframe terminals for making the indicated connections, as well as the connections to gate terminals 16 a-16 d. Optionally, one or more of the interposer sheets 32 a-32 d can extend laterally beyond the outline of the respective transistor chips 12 a-12 d to facilitate electrical interconnections in the liquid cooled package.

FIGS. 2A and 2B respectively illustrate a ½-H-Bridge transistor circuit and a stacked assembly of power-FET chips 12 a-12 d for effectuating the ½-H-Bridge circuit. Referring to FIG. 2A, the letter M designates a two-terminal electrical load such as a DC motor, the “a” terminal of which is coupled to the node 34 of the bridge circuit. The FETs 12 a and 12 b are connected in parallel, and couple the node 34 to power (V+). Similarly, the FETs 12 c and 12 d are connected in parallel, and couple the node 34 to ground (GND). As in FIG. 1A, the source, drain and gate terminals of FETs 12 a-12 d are designated by the letters S, D and G, respectively. In operation, FETs 12 a and 12 b are turned on to connect the load terminal to V+; alternately, FETs 12 c and 12 d are turned on to connect the load terminal to GND.

In FIG. 2B, the reference numeral 40 generally designates a stacked assembly of four power-FET chips 12 a-12 d corresponding to the four FETs 12 a-12 d in the circuit diagram of FIG. 2A. As in FIG. 1B, the letters a-d are appended to the reference numerals designating various elements of the FET chips 12 a-12 d. Also, each FET 12 a-12 d is provided with source and gate terminals 14 a-14 d and 16 a-16 d, channeled drain terminals 18 a-18 d, and a metal interposer sheet 32 a-32 d. The chip-to-chip interconnections in the assembly 40 are the same as in the assembly 10 of FIG. 1B, and are not described again here. In the assembly 40, however, the load terminal a is connected to the drain terminals 18 a and 18 b and the source terminals 14 c and 14 d.

FIGS. 3-4 illustrate a liquid cooled container 50 for housing a stacked assembly of transistor chips 10, 40 such as depicted in FIGS. 1B and 2B. The container comprises a molded base 52 that is recessed to form a fluid chamber 54 in which the transistor chips 12 a-12 d are received, and a molded cover 56 that closes and seals the chamber 54. To this end, the base 52 is provided with a perimeter leadframe element 68 forming a complete path of exposed metal atop the walls bounding the chamber 54 (as well as a package lead), and the leadframe element 68 is soldered to a planar conductor 70 formed on the inboard face of cover 56 to seal the chamber 54.

The floor of the chamber 54 is vertically tiered as seen in FIG. 4 to support the downward-facing surfaces of the chips 12 a-12 d, and a set of leadframe terminals 58 a-58 f insert-molded in the base 52 have exposed surfaces in the floor of chamber 54 that make electrical contact with selected downward-facing terminals of the chip assembly 10, 40. As seen in FIG. 3, the sidewalls 60 and 62 on either side of the vertically tiered floor of chamber 54 are horizontally tiered so that the chips 12 a-12 d individually nest in the chamber 54 with the proper amount of horizontal staggering. The transistor chips 12 a-12 d may be stacked and interconnected as shown in FIGS. 1B and 2B using an external fixture (not shown) and then placed in the chamber 54, or they may be stacked by placing them individually in chamber 54. Referring to FIG. 4, the inboard face of cover 56 is vertically tiered to match the profile of the chip assembly 10, 40, and a set of leadframe terminals 64 a-64 b insert-molded in the cover 56 have exposed surfaces that make electrical contact with selected upward-facing terminals of the chip assembly 10, 40. The leadframe terminals 64 a and 64 b of cover 56 also engage a set of leadframe terminals 66 a and 66 b in the base 52 so that the package leads all extend out of the base 52.

The sidewalls 60 and 62 of base 52 position the stacked chip assembly 10 approximately in the middle of the chamber 54, as seen in FIG. 3, with the channels 28 a-28 d of each chip 12 a-12 d running laterally between the sidewalls 60 and 62. A first fluid passage 69 a formed in the sidewall 60 admits liquid coolant into the chamber 54. The coolant passes through the channels 28 a-28 d of chips 12 a-12 d, and then exits the chamber 54 through a second fluid passage 69 b formed in the sidewall 62.

The leadframe terminals 58 a-58 f and 64 a-64 b of the illustrated container 50 are configured for the H-Bridge chip assembly 10; a somewhat different leadframe configuration would be required for the ½-H-Bridge chip assembly 40. In the illustrated configuration, the base leadframe terminals 58 a, 58 b, 58 c, 58 d, 58 e and 58 f respectively engage source 14 d (load terminal b), gate 16 d, drain 18 c (GND), drain 18 b (load terminal a), source 14 a (V+) and gate 16 a. The cover leadframe terminals 64 a and 64 b respectively engage gate 16 b and gate 16 c, and planar conductor 70 formed on the inboard face of cover 56 engages drain 18 a (load terminal b). The planar conductor 70 is joined to the perimeter leadframe terminal 68 of base 52 as mentioned above, and the terminal 68 is joined to the leadframe terminal 58 a since both are coupled to load terminal b. Once assembled, the various electrical and sealing connections are formed with normal solder in a reducing atmosphere, or with epoxy-based flux solder in a reflow process.

FIGS. 5A-5C simply demonstrate different possible chip-to-chip stack configurations that can be packaged as show or in combination with other chips. In each case, the illustrated interposer sheet 32 can be soldered to either chip 12 a or 12 b. FIG. 5A illustrates a stacked source-to-source transistor circuit, where the horizontal staggering of chips 12 a and 12 b allows bottom-side access to gate 16 a and top-side access to gate 16 b. The drains 18 a and 18 b are also accessible, and the interposer 32 can be extended transverse to the plane of the drawing to afford electrical access to the interconnected sources 14 a and 14 b. FIG. 5B illustrates a stacked drain-to-source transistor circuit, where source 14 a and drain 18 b are accessible, and the horizontal staggering of chips 12 a and 12 b allows top-side access to both gates 16 a and 16 b, as well as the interconnected drain 18 a and source 14 b. Finally, FIG. 5C demonstrates that in a stacked drain-to-drain transistor circuit, access to gates 16 a and 16 b is achieved without horizontal staggering. Similar to the configuration of FIG. 5A, interposer 32 can be extended transversely to afford electrical access to the interconnected drains 18 a and 18 b.

In summary, the present invention provides a cost effective and space-efficient way of forming power electronic circuits with direct die liquid cooling by installing a stacked assembly of interconnected semiconductor chips in a liquid cooled container. While described in reference to the illustrated embodiments, it is expected that numerous modifications and variations in addition to those mentioned herein will occur to those skilled in the art. For example, the interposer sheets 32 can be formed of a dielectric material, or a combination of dielectric and conductive material, to electrically isolate different circuits in an assembly of semiconductor chips, the number of stacked semiconductor chips in a given package may be different than shown, The leadframe terminals of container 50 and cover 56 can be over-molded instead of insert-molded, or fabricated from multi-layer circuit boards. Also, the layout and profile of the chip channels may be different than shown, the number and/or shape of the fluid passages, as well as the number of electrical interconnects per package, may be different than shown, and so on. Accordingly, it is intended that the invention not be limited to the disclosed embodiment, but that it have the full scope permitted by the language of the following claims. 

1. A liquid cooled power electronic circuit, comprising: a sealed container having a chamber with inlet and outlet passages for receiving and exhausting liquid coolant; and a stacked array of semiconductor chips disposed in the chamber, where juxtaposed terminals of said semiconductor chips are electrically joined to define said power electronic circuit.
 2. The liquid cooled power electronic circuit of claim 1, where: the semiconductor chips are vertically stacked with laterally staggering to allow access to selected terminals of said semiconductor chips.
 3. The liquid cooled power electronic circuit of claim 2, where: one or more of said semiconductor chips include first and second terminals formed on a single face thereof, and the semiconductor chips are lateral staggered such that the first terminal is juxtaposed to a terminal of another semiconductor chip, but the second terminal is accessible.
 4. The liquid cooled power electronic circuit of claim 2, where: a floor of the chamber is vertically tiered to support the laterally staggered semiconductor chips.
 5. The liquid cooled power electronic circuit of claim 4, where: the container includes first and second sidewalls laterally bounding the vertically tiered floor of said chamber, and said inlet and outlet passages being formed in said first and second sidewalls.
 6. The liquid cooled power electronic circuit of claim 5, where: said first and second sidewalls are laterally staggered to define a predetermined relative positioning of said semiconductor chips.
 7. The liquid cooled power electronic circuit of claim 5, where: a major surface of each semiconductor chip is channeled, and the semiconductor chips are each oriented so that the channels run laterally between the first and second lateral sidewalls to conduct liquid coolant between the inlet and outlet passages.
 8. The liquid cooled power electronic circuit of claim 1, where: said container includes a base and top sealed to said base; said base includes a first set of leadframe elements that engage semiconductor chip terminals that face a floor of said chamber; and said top includes a second set of leadframe elements that engage semiconductor chip terminals that face said top.
 9. The liquid cooled power electronic circuit of claim 1, where: the stacked array of semiconductor chips includes interposer sheets disposed between said juxtaposed terminals.
 10. The liquid cooled power electronic circuit of claim 9, where: one or more interposer sheet is formed of dielectric material to electrically isolate stacked semiconductor chips separated by the interposer sheet.
 11. The liquid cooled power electronic circuit of claim 1, where: said semiconductor chips are transistors, and said power electronic circuit is an H-Bridge or a ½-H-Bridge. 